lift bag

In Scuba diving, a Lift Bag is used to raise an object heavier than about 4kg/8lbs to the surface. It is usually a nylon bag with a bladder inside. You inflate it with an air source to establish positive buoyancy.

You use a lift bag as exertion under water can be dangerous due to increased risk of Decompression Sickness. Using your own BCD to lift heavy objects is bad practice: If the item falls, your buoyancy will take you up at an uncontrollable rate, which is very dangerous.

Due to Henry's Law, the air you put into the lift bag will expand as you get to shallower water (Every 10 meters of ascent, the volume of air in the lift bag will double). To control the ascent with your item, the lift bag has a dump valve built in, both to prevent it from tearing open if it were to be stretched to its limit, and to help divers take air out of the lift bag when ascending.

Using a lift bag is deceptively tricky. The ideal is to fill the lift bag just enough for an item to achieve neutral buoyancy, then slowly swim it to the surface, dumping little bits of air from the lift bag to maintain neutral buoyancy.

How big a lift bag will I need?

Say you have a 50kg outboard motor than you want to raise to the surface. It displaces 20 litres of sea water.

A litre of sea water weighs 1.03 kg, and the engine already displaces 20.6kg worth of water, so it's 29.4 kg negatively buoyant. For the remaining 29.4 kg, you must displace 28.5 liters of sea water, so you're going to need a 30 litre lift bag in order to lift the item from the bottom of the ocean.